Stanford Engineers Invent All-Carbon Solar Cell

We've told you how carbon nanotubes are being used for some amazing things, like the basis of a wearable energy-harvesting fabric, and reinforcing carbon fiber composite to make it seaworthy for duty as a boat construction material. Now, in a major first, researchers at Stanford University have built a prototype of an all-carbon solar cell, including carbon nanotubes.

Why carbon? Because it's a lot cheaper than silicon, and it's also abundant and highly conductive.

Carbon has the potential to produce high performance for its low cost, said Stanford Professor of Chemical Engineering, and the study's senior author, Zhenan Bao, in a university news article. "Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties."

The study builds on previous work done in her research group's lab. The results were published in an article in ACS Nano (subscription or purchase only).

Researchers at Stanford University have built a prototype of an all-carbon solar cell that includes carbon nanotubes in both the photoactive layer and the electrodes. (Source: Mark Shwartz / Stanford University)

The prototype is a flexible, thin-film solar cell made of carbon that is coated from solution. In most thin-film solar cells, the electrodes are made of conductive metals, such as silver and indium tin oxide. But as demand for indium rises, it's becoming scarce and more expensive, compared to carbon.

The new carbon-based cell consists of a photoactive layer made of carbon nanotubes and 1-nm-wide carbon buckyballs, sandwiched between two electrodes. The electrodes are made of single-walled carbon nanotubes and graphene.

The technique used for coating the cell can further reduce manufacturing costs, since compared to processing silicon, the team's coating methods reduce process steps and don't require expensive tools and manufacturing equipment, said Stanford graduate student Michael Vosgueritchian, co-lead author of the study. Potential applications could be coating windows, cars, or the surfaces of buildings to capture solar energy and generate electricity.

Because carbon withstands extreme environments much better than silicon, these cells might also be used in high temperature, high stress applications where silicon can't function.

"To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon," said Bao. Although other researchers have reported the production of an all-carbon solar cell, those prototypes' carbon included only the inside photoactive layer, and not the electrodes.

Although carbon is plentiful and highly conductive, it's not known for being a good semiconductor, and is therefore very inefficient, since it absorbs near-infrared (NIR) light wavelengths. However, single-walled carbon nanotubes have superior electrical and mechanical properties, and many of them are semiconducting. They can also be easily dispersed in organic solvents or water, making them compatible with high-volume, low-cost coating processes such as large area roll-to-roll.

The research team is examining different methods for improving efficiency. One way is to improve the smoothness of each layer by improving layer-stacking methods, so they can collect more current. Other carbon nanomaterials that absorb more light across a wider spectrum are also being considered.

Other members of the research team included co-lead author of the study, postdoctoral Stanford researcher Marc Ramuz; postdoctoral Stanford researcher Peng Wei; and Chenggong Wang and Yongli Gao of the University of Rochester's Department of Physics and Astronomy.

Earl, thanks for the comments on alternative energy and space exploration. I didn't realize that alt energy projects/technologies had been developed in that context, but it sure makes sense. The high and continually rising price of indium and its projected scarcity, as mentioned in the article, is a major reason the researchers were interested in developing a carbon alternative. No problem re length--we love long comments! (BTW, my name is Ann).

Hello again Amy and tech oriented correspondents: I did as Amy suggested and found more material on subjects I am interested in. Some background: I have been interested in "alternative energy" since the 1960s. This is in part due to my long term interest in space exploration. There was a very large amount of practical ( for powering space systems version of practical) and advanced research done on systems that could be used for space power plants that we thought we would need in the relatively near future. This included thermoelectric and thermo ionic sources ( for Venus and Mercury probes Molybdenum Silicide thermoelectric conversion elements as an example). I also read about the work on Earth based applications, such as water heating devices that was mentioned as a desirable, economic way of using the Sun, during the 1970s and onward. This is a significant source of water heating for homes in Japan. As for the carbon based cells versus the present units with rare elements: I have looked into the various material used in Silicon and other cells ( and L.C.Ds. as well) and found that Indium, courtesy of Indium Corporation of America, is somewhat pricy except in thin films: a retail price for a pound ( from a Mc Master Carr catalog) was ~$350/ pound. Sorry for the long message,but, I think these points might be of interest to your audience, and you, Amy.

Thank you, Earl.

P.S.: I am intersted in energy beamed from space and have been on panels, and attended talks, on this subject.

scifi tech guy, I like your handle :). Regarding efficiency, that's a good question: it can be measured in several different ways. Also, this is a prototype, not a working product, and definitely not a product that's been transferred to a volume manufacturing process. Check out the discussion below: many of the comments concern the subject of efficiency.

Hi: Very nice summary, and, I am interested in the topic. What I would like to know however is what the efficency is of the device. I know it is extremely early in its' developement, but, I like to know such things. This is extremely early: I remember when a high efficency cell was a fantastic 5% because it was that new silicon stuff!

Charlie, you took the words right out of my mouth. As a consumer, I am continually frustrated by the apparent inability of manufacturers to give performance specs about their products in terms that not only make sense to me, but that are actually usable in an on-the-ground kind of way, not abstract numbers. My anti-favorite one is "joule", used by home office UPS makers. I can never, ever remember what it means and when I look it up, it still means nothing in terms of my home office equipment. But the first thing UPS makers ask is "how many joules do you need?" Sure, I'm an electrician and I think in joules every day, uh-huh. No guys--you're supposed to tell me how many I need based on the info I can provide you.

Earlier posts mentioned 'efficiency' being the #1 issue. While that may be true, it begs the question: How do you measure efficiency? In a spacecraft, it's watts per sq in & watts per pound. But for an individual user like me who wants to make electricity for my home & vehicles, that's meaningless. The only thing I care about is watts per dollar. If this tech can reduce environmental costs, materials costs, production costs, and installation costs (all 'per watt'), that's where I'll put my money.

It's also the case that storing heat for heating water--in large sealed water bottles, rocks and even earth walls--is a lot easier to do than storing "energy" in some other form. It's also been done already.

Akwaman--You are absolutely correct. Water heating is a significant factor in energy usage and solar water heating is definitely one viable solution to that problem--when possible. Years ago, I worked for a water heater manufacturer and one item in our product line was a water heater using solar roof-top panels to "collect" the sun's rays and provide for heating. We had auxilary heating elements when needed during inclement weather. The issue in the southeast was considerable cloud cover that negated available sunlight--and lengthened the ROI. Any advancement such as the one Ann is describing is definitely welcomed to that particular industry. As advancements in solar technology progress, we will see added sales and resurgence in usage even in the most difficult environmental situations.

I especially like the part about the elimination of indium and silver in this process. As exotic and rare minerals become hard to procure (and with their unequal distribution for each country), this characteristic will make this option more and more attractive.

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.